1. Field of the Invention
This invention relates to a zoom lens of a high power varying ratio and more particularly to a zoom lens which is highly suited for a photographic camera, a video camera or the like having a high power varying ratio and a high optical performance over the whole variable power range thereof.
2. Description of the Related Art
A zoom lens having a high power varying ratio with a short total length has heretofore been desired for a photographic camera, a video camera, etc.
U.S. Pat. No. 4,749,266, filed Nov. 16, 1983, corresponding to Japanese Patent Application Laid-Open No. SHO 59-93411, has disclosed a zoom lens which is composed of five lens units or groups and has a high magnifying power varying ratio up to 10 by shifting four of these lens units or groups. In this case, the five lens units are arranged to have refractive powers in the order of positive, negative, positive, negative and positive refractive powers as viewed from the side of an object to be photographed. The zoom lens disclosed is arranged to have a high optical performance over the whole variable power range thereof with aberration fluctuations which take place in varying the power of the lens adequately corrected by specifically setting the shifting conditions of the lens units and their refractive powers.
In the case of the zoom lens disclosed in the patent application cited above, the telephoto ratio (the ratio between the total lens length obtained at the wide-angle end of the lens and the focal length obtained at the telephoto end) is 1.35 and the power varying ratio is 10. The total lens length is relatively short for a zoom lens. Besides, the F number is 1.6. The lens thus has a large aperture ratio and is particularly suited for a video camera.
U.S. Pat. No. 4,437,732, corresponding to Japanese Patent Application Laid-Open No. SHO 57-2014 has disclosed a zoom lens. The zoom lens also includes five lens units or groups having their refractive powers arranged in the order of positive, negative, positive, negative and positive refractive powers as viewed from the side of the object. All the lens units or groups are arranged to be shifted under a predetermined condition in varying the magnifying power of the lens. However, the lens has a power varying ratio of only 4.5, which is not sufficient.
Meanwhile, there have been proposed various zoom lenses of the kind called a rear focus type which are arranged to shift lens units or groups other than a first lens unit or group disposed on the object side. An example of them is disclosed, for example in Japanese Patent Application Laid-Open No. SHO 58-136012.
Generally, the rear focus type zoom lens is arranged to shift relatively small and light-weight lens units or groups. Therefore, a focusing action can be promptly carried out with a small lens unit driving force. However, with the focusing action performed by shifting the lens units which are located in the rear of a power varying lens unit in the rear focus type zoom lens, the zooming position of the lens sometimes varies for the same object distance. In other words, the drawing-out (or delivery) extent of the focusing lens unit varies accordingly as the focal length varies and the drawing-out extent sometimes quadratically or noncontinuously varies.
In the zoom lens of this kind, a larger space must be provided on the wide angle side to allow the focusing lens units to shift in order to have a higher power varying ratio. Then, this results in an increase in the size of the lens system. Further, with the above stated rear focus type zoom lens employed, the drawing-out extent of the focusing lens unit on the telephoto side sometimes becomes two or three times as large as the drawing-out extent on the wide angle side for the same object distance.
Further, in the zoom lens of this kind, the sensitivity, which means the shifting degree of an image surface in relation to the shifting extent of the focusing lens unit, increases on the telephoto side. If the value of this sensitivity increases to exceed a certain degree, mechanical shift control over the focusing lens unit becomes difficult. However, if the sensitivity obtained on the telephoto side is set at a controllable value, the sensitivity at the wide angle end of the lens becomes too low, thus requiring a larger space for shifting the focusing lens unit. Then, this result in an increased size of the lens system.
A further example of the prior art relative to this invention is disclosed in U.S. Pat. No. 4,636,040, corresponding to Japanese Patent Application Laid-Open No. SHO 58-202416.